At first I'd post to the official addon comments so the addon author notices it and may answer you:
https://www.esoui.com/downloads/info....html#comments
Maybe someone else got the same problem there and asked + got a question so search the comments.
In addition read the offuicial websites FAQ, maybe it answers the problem there already:
https://www.tamrieltradecentre.com/help/FAQ
And allow/whitelist/add to exclusion list Tamriel Trade Center client.exe and the addon's folder in your antivirus/firewall (Windows Defender & Firewall, Sophos, Avast, Avira, McAffee, etc.) so the client and SavedVariables folder is able to communicate with the server of TTC properly.